In a current computed tomography system, an x-ray source projects a fan-shaped beam that is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the “imaging plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a display.
Dual-source or dual-energy CT systems have two separate x-ray sources and associated detector arrays, which rotate together in the gantry during a scan. The x-ray sources may be operated at different energy levels to acquire two image data sets from which a low energy and a high energy image may be reconstructed.
Dual-energy CT systems are typically used for specific applications, such as bone removal, iodine quantification, and material characterization. In these applications, the two data sets at low- and high-energy are acquired simultaneously, which eliminates the mis-registration problems. Many dual-energy processing techniques, either before or after the reconstruction, can thus be applied on the low- and high-energy data sets to obtain basis material-specific information.
In addition to the “specialized” images that are generated in these applications, another set of images is often generated from the dual-energy data to provide images for “traditional” diagnostic purposes. Since “traditional” diagnostic images are typically generated using a single-energy CT system, an operator typically selects either the low- or high-energy dataset to generate a set of images. However, this method creates images with reduced image quality when compared with an image generated from a “single-energy” data acquisition because each of the image data sets from the dual-energy scan is generated with approximately one-half of the radiation dose of the corresponding single-energy scan.
Accordingly, some operators have been known to perform a second, single-energy data acquisition from which to generate the desired images. However, this has a number of drawbacks. First, it subjects the patient to additional doses of radiation. Second, the dual-energy datasets and the single-energy datasets are not temporally related and may be mis-registered.
Therefore, it would be desirable to have a system and method for providing an operator with a variety of images that convey the advantageous information included in both dual-energy images and single-energy images without the need to expose the subject to multiple imaging acquisitions and the separate radiation doses associated with each acquisition. Furthermore, it would be desirable that such a system and method reduce the likelihood of temporal and spatial mis-registrations.